Crack Paths 2006

Therefore, in 1974 some tests [14], were carried out at room temperature on Waspaloy,

a nickel based gas turbine material, in order to determine the conditions under which a

fatigue crack path became unstable under biaxial loading. The specimens were 254 m m

square and 2.6 m mthick. The material had been cross rolled during production to

ensure that its properties were reasonably isotropic. Tests were carried out using

sinusoidal constant amplitude loading at a stress ratio (ratio of minimumto maximum

load in fatigue cycle), R, of 0.1. In each test the fatigue load perpendicular to the crack

was kept constant. Cracks were first grown from each end of an initial slit under

uniaxial loading. An in phase load was then applied parallel to the crack, and crack path

behaviour observed. Figure 6 shows the crack path for a load parallel to the crack of

twice the load perpendicular to the crack. The crack path became unstable and deviated

from its initial path as soon as the load parallel to the crack was applied. At the time the

tests were carried out it wasn’t possible to do more than describe the results. However,

reanalysis of these and other results in 1997 [4, 15] showed it was possible to correlate

crack path stability in terms of a parameter called the T-stress ratio.

PLASTIDC O M E S T ITCA P

Figure 7. Plastic domestic tap.

Figure 8. Crack surface of plastic

domestic tap.

In 1991 a plastic domestic tap in the author’s utility room was observed to be leaking

where it was screwed into a fitting on the supply pipe. The tap had a fitting for a hose

pipe, and appeared to be a replacement for the original brass tap. Whenan attempt was

made to unscrew the tap it failed completely. The two parts of the broken tap are shown

in Figure 7 and a close up of the fracture surface in Figure 8. The dark area is fatigue

and the light area the final static failure. The age of the tap at the time of failure is

unknown, but as one fatigue cycle is applied each time a tap is turned on and off it is

likely that thousands of cycles had been applied. Safety critical pressure containing

components are often designed to leak-before-break [6] in order to avoid catastrophic

failure. It is fortunate that the tap did so otherwise the utility room would probably have